Objectives: We aimed to indicate simple determinants of abnormal body composition in children, such as socio-economic status (SES), duration of breastfeeding, parental age and birth parameters. Methods: The final data set consisted of 469 healthy prepubertal individuals (247 girls and 222 boys). We studied body mass, body height, and parameters of body composition such as muscle mass and fat mass. The birth parameters and gestational age were obtained from the children's medical record books held by the parents which were completed by medical personnel immediately following birth. Information about socio-economic status (SES), duration of breastfeeding and parental age was obtained by questionnaire. The statistical methods included forward multiple regression and generalized linear models (GLZ) or general linear model (GLM). Results: Higher fat mass (FM) (%) was connected with shorter duration of breastfeeding (< 2 months and lower SES (p < 0.05). Lower muscle mass (MM) (%) was linked with lower SES (p < 0.05) and lower birth weight (p < 0.05). Higher body mass index (BMI) was connected with higher birth weight (p < 0.05), shorter duration of breastfeeding (< 2 months) and lower SES (p < 0.05). Moreover interaction effects were observed in the case of the FM (%) (breastfeeding x SES; breastfeeding x parental age) and the BMI (breastfeeding x paternal age). Conclusions: Body composition can be linked with the duration of breastfeeding, SES, parental age, birth weight and birth length.